This invention relates to a method of controlling the operation of a Francis type pump turbine, more particularly a method of controlling the operation of the pump turbine when the operation of a dynamoelectric machine, acting as a synchronous generator or a motor, and directly coupled with the runner of the pump turbine is switched between a generator operation in which the dynamoelectric machine is connected to an electric power system to operate synchronously therewith and a synchronous condenser operation.
In a Francis type pump turbine, it is often operated as a water turbine under no load condition so as to operate the dynamoelectric machine as a synchronous condenser for controlling the power factor or stability of the power system. To operate the dynamoelectric machine as a synchronous condenser, it is connected to the power system, and the guide vanes and an inlet valve are fully closed while at the same time compressed air is admitted into the runner chamber to force down the water level therein to a predetermined level in a draft tube so as to rotate the runner in air thus decreasing a reaction torque acting upon the runner.
To switch the operation of the dynamoelectric machine from the condenser operation to the generator operation while the generator is being connected to the power system, the inlet valve is closed and the compressed air in the runner chamber is released to cause the runner to rotate as a water turbine in a shut off state. Thereafter, the guide vanes are opened to pass water through the runner to initiate power generation, the quantity of water being controlled by the degree of opening of the guide vanes.
Among hydraulic machines, the runner blades of the Francis type pump turbine are designed such that the centrifugal force effect of the runner against the water flow would be large for pumping up water when it is operated as a pump. For this reason, even in a turbine operation, the pressure in a runner chamber would be increased due to a large centrifugal force of the rotating runner under a shut off state or small water flow quantity state near no load condition so that water will be forced into a spiral chamber from the runner. When the inlet valve is fully opened, water can be pumped up. Where the characteristic of torque T acting upon the runner for respective degrees of opening of the guide vanes is expressed in terms of the relative relationship between a unit torque per one meter head T/H and a speed per one meter head N/.sqroot.H (where N represents the speed of the water wheel, H the operating head), the characteristic curve of the unit torque per one meter head T/H of a Francis type pump water tends to bend in a direction in which the unit speed per one meter head N/.sqroot.H decreases near no load (zero torque condition) and then rapidly drops into a negative torque region as shown in FIG. 1. In FIG. 1, a curve a.sub.0 represents the fully opened state of the guide vanes and has the following relation with respective degrees of opening EQU a.sub.0 &lt;a.sub.1 &lt;a.sub.2 . . . a.sub.7.
In FIG. 1 when the hydraulic machine is operated as a turbine after determining the operating head H.sub.0 on the low head difference side where the torque characteristics bend greatly under a rated rotating speed N.sub.0 the relationship between the degree of opening a of the guide vanes and the torque T is shown by a graph shown in FIG. 2. With a hydraulic machine having such characteristic when the operation of the turbine is switched to a generator operation from the shut off operation, when the guide vanes are opened after fully opening or sufficiently opening the inlet valve according to the prior art method, characteristics as shown in FIG. 3 are obtained. In this case, as shown by the uppermost graph shown in FIG. 3, with increase in the degree of opening of the guide vanes the running state changes such that, after falling into a large negative torque region to reach a lowest state B, then to a desired generator running state (positive torque) after passing through a no load (torque) running state C. Thus, although the running state should be rapidly switched to the generator operation when the power demand increases, according to the prior art method by which the water flow is adjusted only with the guide vanes, as the hydraulic machine must pass through an extremely large negative torque region, not only a large electric power is consumed but also the load of the generator rapidly varies, thus causing a surge or power fluctuation in the electric power system.
For this reason in a hydraulic machine having a bent torque characteristic as above described, while the generator is being connected to the power system when the operation is switched from the synchronous condenser operation to the generator operation it is necessary to provide control apparatus capable of smoothly switching the operating state of the generator without passing through a negative torque region.
The problem of the negative torque caused by the opening of the guide vanes also occurs when the generator is switched to the synchronous condenser operation from the generator operation.